The field of the present invention is conveying and metering pulverulent, dry solids and the present invention is particularly concerned with chemical processes in which the solid should be metered into an apparatus in which there is a superatmospheric pressure of a gaseous system. At the same time, a sure sealing against the surge chamber is needed in this case. But the task is made especially difficult when the gas chamber under superatmospheric pressure has an elevated temperature and contains the vapor of a liquid, which vapor would cool off in the conveyed powder upon flashing back and would condense in the powder.
For example, such a problem exists when terephthalic acid (TPA) and methanol (M), which are solid and liquid, respectively, at normal conditions, are to be esterified to terephthalic acid dimethyl ester (DMT) at an elevated temperature in the gas phase. The state of the art of methods and apparatus for the esterification of TPA and M to DMT may be ascertained by reference to British Pat. No. 1,305,290 and U.S. Pat. Nos. 3,617,226; 3,886,200; 3,940,431; 3,972,912; 3,980,431 and 4,015,943, the disclosures of which are incorporated herein. During these processes, as disclosed in the British and U.S. Patents, pulverulent TPA along with M are fed into a rotary furnace, fluidization oven or preliminary reactor, which are at an elevated temperature and under increased pressure, and the reactants are esterified, partly directly, partly in devices connected at the outlet side, wherein the gaseous reaction material can also partly be circulated. In any case, pulverulent, dry TPA is metered into an apparatus containing a hot gas mixture made from TPA, M, DMT, terephthalic acid monomethyl ester and water. The apparatus of U.S. Pat. Nos. 3,980,441 and 4,015,943 are especially advantageous; since M is used here only in a slight excess, but at the same time the work is conducted at 300.degree. to 340.degree. C. at a guage pressure of 1 bar, the uniform and reliable conveying and metering of the TPA becomes especially important.
In the prior art dry, pulverulent material is metered into an apparatus containing a hot gas mixture by conveying a suspension of TPA in M, e.g. with the aid of a centrifugal pump. Instead of vaporizing the M and bringing it up to the required reaction temperature before its entry into the apparatus, the heat is applied by heat exchange surfaces which are arranged inside of the reaction system and during this process, these surfaces quickly become encrusted with TPA which is hard and lumpy even though constantly being scraped off by rotating scrapers and it eludes the desired reaction.
The prior art pneumatic conveyance is also elaborate and unsatisfactory, as the solid and the conveying gas must be in a determined mass ratio, as the entire system must be heated because of the M, and as a corresponding inlet pressure of the conveying gas must be maintained. Therefore, a metered feeding device for the TPA against the pressure system is necessary, e.g. a screw or a bucket wheel under a pressure provided in the heated TPA container, wherein the gas seal can be attained with the aid of bucket wheel charging units or double flap valves with pressure equalization. In all of these devices in which TPA is periodically fed in and discharged, it is unavoidable that vaporous M penetrates backwards and causes disturbances of all sorts. But particularly the exact continuous adjustment of a mass flow of pulverulent solid is not the concern, but rather the discontinuous periodical metered feeding of small quantities.
Thus the essential prerequisite of many commercial processes which require the constant conveying and metering of a powder remains unfulfilled. In the selected example, the only way to attain that the TPA is vaporized free from residue and completely esterified in dependence on the residence times, the concentration ratios and the temperature profiles based on the mass ratios, is by the continuous metered feeding of both reactants, so that the given arrangement is made maximal use of and the reaction mixture obtained can be simply and constantly worked up.